Quote from DPReview:
"The big problem with this lens, however, lies in the focusing systems. The autofocus motor is relatively slow and noisy, which puts it at a major disadvantage compared to its ultrasonic motor-equipped competitors, simply because AF performance impacts strongly on many of the typical uses for a fast telezoom lens. The slow speed of the motor means focus tracking simply isn't fast enough for sports or action, or even children running around playing; it's also a distinct problem in low light or with low-contrast subjects, where the lens can take what feels like an eternity to achieve focus. This is compounded by the noise the motor makes, which could be a real problem in situations such as wedding or event photography for which the user wishes to remain unobtrusive; discrete it is not. Also the Tamron lacks a focus limiter switch, so has a tendency to hunt back and forwards through its entire range when it fails to lock focus first time. But perhaps the biggest problem we encountered was a tendency for the lens to mis-focus, seemingly at random and disturbingly frequently, such that F2.8 shots in particular were often not critically sharp (although it must be pointed out that this may be specific to either our test sample, or the Canon mount version).
However it's not just the autofocus system which is an issue, and the manual focus isn't perfect either. The use of the push-pull clutch on the focus ring to engage manual is a good idea in principle, but not perfectly implemented in practice; the action is remarkably noisy (in context, the click-clunk noises on switching between AF and M are substantially louder than the shutter release on any current DSLR, including the top-end professional models such as the Nikon D3 and Canon EOS-1Ds Mark III), and the focus ring has a bad habit of sticking in an intermediate position in which the gearing isn't correctly engaged, and manual focus doesn't work properly. Also, it's not possible to use autofocus to pre-focus on a specific point then switch to manual, as the focus distance can often be thrown in the switchover. Finally, the manual focus ring is just a little too highly geared, with relatively short travel between infinity and the close focus of 0.95m, making critically accurate manual focus difficult to achieve (certainly outside of live view)."
This is an area that would greatly concern me... Your mileage may vary...
R.
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